This invention relates to a method of precipitating microfine silica. More particularly it involves preparation of silica that has phosphate ions in the surface and is therefore readily dispersible.
The preparation of finely divided or microfine silica, 10 to 1000 mu, is well known and numerous patents have issued describing such processes. An example of such a patent is U.S. Pat. No. 3,208,823, hereby incorporated by reference. This patent describes methods of preparing finely divided hydrated silica by forming a protocoacervate from a soluble silicate solution using coacervating agents such as highly soluble salts, especially univalent inorganic salts; completely water miscible hydrogen bonding agents such as simple alcohols and ketones, and highly soluble nitrogen bases such as ammonia and amines. This and other precipitation methods using a coacervation step involves formation of a highly alkaline phase some of which will be in the surface of the coacervated micelles and can be formed into gel when the micelles are precipitated as silica. This gel structure prevents the use of such precipitated silica in certain applications.